Add export / import function for config (Android app)

[cmeis]

Add the ability to export / import the app config (settings and mainly subscribed topics) for backup purposes.

[binwiederhier]

I like this idea. It annoys me a bit myself.

[binwiederhier]

This is done as part of binwiederhier/ntfy-android@8e1830d. Will be in the next release of the Android app